1. Field of the Invention
The present invention relates to a projection display apparatus with colored light combining means and a lighting optical system therefor.
2. Discussion of the Background
A cross dichroic prism is often used for projection display apparatus that project a color image on a projection screen. For example, in a transmissive liquid-crystal projector, the cross dichroic prism is utilized as colored light combining means that combines three colored rays of red, green, and blue and emits the composite light in a common direction. In a reflective liquid-crystal projector, the cross dichroic prism is utilized as colored light separation means that separated a beam of white light into three colored rays of red, green, and blue and also as colored light combining means that recombines modulated three colored rays and emits the composite light in a common direction. A known example of the projection display apparatus with the cross dichroic prism is disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 1-302385.
FIG. 17 conceptually illustrates a main part of a projection display apparatus. The projection display apparatus includes three liquid-crystal light valves 42, 44, and 46, a cross dichroic prism 48, and a projection lens system 50. The cross dichroic prism 48 combines three colored rays of red, green, and blue modulated by the three liquid-crystal light valves 42, 44, and 46 and emits the composite light toward the projection lens system 50. The projection lens system 50 focuses the composite light on a projection screen 52.
FIG. 18 is a partly decomposed perspective view illustrating the cross dichroic prism 48. The cross dichroic prism 48 includes four right-angle prisms which are bonded to one another via the respective right-angle surfaces by an optical adhesive.
In the projection display apparatus using a cross dichroic prism as the colored light combining means, according to the optical characteristics of a light source applied, scattering of light at a joint of four right-angle prisms may cause a dark shadow on a projected image.
FIG. 19 shows a problem arising in the case of utilizing the cross dichroic prism 48. As shown in FIG. 19(A), the cross dichroic prism 48 has a red light reflection film 60R and a blue light reflection film 60B which are arranged in a substantially X shape on an X-shaped interface formed by the right-angle surfaces of the four right-angle prisms. There is a layer of optical adhesive 62 formed in the gaps between the four right-angle prisms. Both the reflection films 60R and 60B accordingly have gaps at a central axis 48a of the cross dichroic prism 48.
When a light beam passing through the central axis 48a of the cross dichroic prism 48 is projected on the projection screen 52, a dark line due to the central axis 48a may be formed in the projected image. FIG. 19(B) shows an example of the dark line DL. The dark line DL represents a relatively dark, linear area having a different color from that of the other part and is formed substantially on the center of the projected image. It is considered that the dark line DL is ascribed to scattering of rays and no-reflection of the red light and blue light in the gaps of the reflection films in the vicinity of the central axis 48a. A similar problem arises in a cross dichroic mirror that includes two dichroic mirrors that are arranged in an X shape and respectively have selective reflection films, such as a red reflection film and a blue reflection film. In this case, a dark line due to a central axis of the mirror is formed in a projected image.
As described above, in the prior-art projection display apparatus, a dark line is formed substantially on the center of a projected image because of the central axis of the cross dichroic prism 48 or the cross dichroic mirror.